Two blustery weather fronts - one warm, one cold - blew through the Washington area Friday, lifting hats, umbrellas and at least one rooftop and knocking out power to thousands of residents.

Trees were reported down along the northern stretch of the George Washington Parkway and in Northwest Washington. And people living in Northeast Washington were evacuated after winds blew part of the roof off of their three-story apartment building.

In Virginia, high winds tore the face off a sign for Potomac Mills mall, off Interstate 95 in Woodbridge, for the second time in recent days, the Associated Press reported. The part of the sign facing southbound traffic came off Friday afternoon. The facade facing northbound traffic tore off last Saturday, when winds reached 65 mph.

A mall spokeswoman told the Associated Press that on both occasions, pieces of the sign fell near guardrails on I-95.

Metro reported delays on the Red Line for a time after a pole fell and struck an inbound train near the New York Avenue station. No injuries had been reported.

Virginia Railway Express trains on the Manassas Line experienced delays due to a downed tree near Backlick Road. And there was signal trouble on MARC's Brunswick Line, although it was not clear whether it was weather-related.

There were also delays at airports across the Northeast amid the thunderstorms, hail and sunny skies that were reported on and off as the two fronts marched across the D.C. area. A high-wind warning was in effect until 7 p.m. Friday.

Some arriving flights at Dulles International Airport were delayed almost two hours because of wind. Jason Samenow of The Washington Post's Capital Weather Gang reported that a 58 mph gust was clocked at Dulles.

Several flights heading to New York and Philadelphia area airports were delayed more than three hours, according to the Federal Aviation Administration.

In Rockville and Bethesda, power outages left traffic lights and at least one local business without power.

At a Giant supermarket in Bethesda, cash registers were down and employees were using calculators to check out long lines of customers, who could only pay with cash.

Elsewhere in Montgomery County, the Aspen Hill, Potomac and Quince Orchard libraries closed at 5 p.m. because of power outages.

In addition to the Potomac Mills sign, last Saturday's wind also wind blew up some nasty brush fires across the area and knocked down the 30-year-old National Christmas Tree, south of the White House.